Because of the strictness of different electrical and mechanical requirements, the production of wave-guide elements, especially cavities and filters of rectangular wave guides, represents a highly laborious process being consumptive of time, material and machine capacity.
The contradictory mechanical and electrical requirements are due to the fact that on the inner conducting surfaces of the finished wave-guide element should have a continuous electroplated coating of high-grade surface finish preferably of noble metals (Ag, Au, Pd), positioning of the components should be kept within strict limits of tolerance, formation of a continuous tight electroplated coating between the current-conducting surfaces joined is imperative, but yet perfect compensation of thermal expansions should be provided for.
In production technologies used up to now the inner continuous electroplated coating of uniform thickness has been formed after the wave guide element was given its final form. Formation of an inner layer of uniform thickness inside the rectangular wave guide provided with corners, projections and transverse iris plates and rods, however, proved to be nearly impossible in practice. In order to perform suitable electroplating, inner equipotential surfaces ought to be formed, but due to the hollow form closed on all sides, formation of equipotential surfaces is impossible. As a consequence, to achieve the desired thickness even in the most unfavorable places, e.g. in corners, thicknesses of noble metal are electroplated, the thickness of which are 10-20 times greater than needed. This also applies to priming galvanic coatings, e.g. copper, under the noble metal coatings. The process mentioned results in significant consumption of noble metals, and taken up electroplating equipment capacity for an unnecessary long period.
In order to comply with the requirement of continuous and smooth inner surface, in the course of producing wave-guide elements, the number of joining operations is minimized.
The wave guides can be made of tubes having different profiles (e.g. circle, ellipse or rectangle or combination thereof same) in which highly accurate slots are machined for the fitting of the transverse iris plates. In the course of mechaning, the inner surfaces of the cavities must remain absolutely free from burrs. In case burrs appear they must be removed without scratching the inner surface. Difficulties in production are increased by the fact that in order to lessen thermal expansion the parts are made of metals having a low thermal expansion coefficient, advantageously of a alloy containing 36% nickel and 64% iron, requiring special bonding technology.
Iris plates are fixed in such a manner that between the inner surface of the tube and the current-conducting surfaces of the iris plates a continuous metallic bond is established. Up to now this requirement could not be complied with. Most frequently brazing with alloys containing silver are performed. With such brazing, it can not be totally ensured that the brazing alloy will fill up all gaps. Simultaneously excessive flow of the melted alloy into the inside of the tube should be avoided.
Furthermore because of the closed inner space, the control of the joint quality of brazed wave-guide elements is practically impossible.
For the reasons enumerated and because of the oxidizing, deforming and surface-roughening effects of conventional welding methods, welding technology has not been used up to now for the assembly of wave-guide elements.